help_heading doesn't seem to work

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Initially reported as #1954

Code

use clap::{App, Arg};

fn main() {
    App::new("app")
        .arg(Arg::from("--flag").help_heading(Some("CUSTOM")))
        .print_help()
        .unwrap();
}

Steps to reproduce the issue

1. Run cargo run

Version

• Rust: 1.43.1
• Clap: 3.0.0-beta.1

Actual Behavior Summary

app 

USAGE:
    app [FLAGS]

FLAGS:
        --flag       
    -h, --help       Prints help information
    -V, --version    Prints version information

Expected Behavior Summary

app 

USAGE:
    app [FLAGS]

FLAGS:
    -h, --help       Prints help information
    -V, --version    Prints version information

CUSTOM:
    --flag

Additional context

help_heading doesn't seem to work at all. I checked flags, options and positional args, incuding multiple options and combinations.